Null steering is a procedure which has been known for many years and is described, for example, in such typical articles as "Adaptive Antenna Systems", by B. Widrow et al, Proceedings of the IEEE, Volume 55, No. 12, December, 1967, and "Adaptive Noise Cancelling; Principles and Applications", by B. Widrow, et al, Proceedings of the IEEE, Volume 63, Number 12, December, 1975. In one of its common forms, null steering is a technique whereby a main antenna and one or more secondary antennas are utilized, and the secondary antenna signals are weighted and summed together with the main antenna signal to form a composite antenna pattern in which undesired signals are removed. The pattern is formed in such a manner as to create antenna pattern nulls in the direction of undesired signals, such as jamming signals and the like, and the main lobe is defined by the characteristics of the main antenna. Using null steering techniques, nulls on the order of 50db can be automatically steered in the directions of jamming signals while the main lobe of the main antenna remains, theoretically, relatively unaffected.
The weighting of the secondary antenna signals to provide an output signal having the correct phase and amplitude for cancelling undesired signals in the summing device has been accomplished with a variety of null steering devices, or circuits, in the prior art. One typical null steering circuit is disclosed by P. W. Howells in U.S. Pat. No. 3,202,990, entitled "Intermediate Frequency Side-Lobe Canceller", issued Aug. 24, 1965. Somewhat different null steering circuitry for adjusting the phase and amplitude of the undesired signals is disclosed by Raymond J. Masak in U.S. Pat. No. 3,981,014, entitled "Interference Rejection System for Multi-Beam Antenna", issued Sept. 14, 1976. In U.S. Pat. No. 3,177,489, entitled "Interference Suppression Systems", issued Apr. 6, 1965, B. Saltzberg discloses a slightly different type of null steering circuit.
The main difficulty with the prior art null steering devices is that main lobe signals which reach the secondary antennas cause nulling or degrading of the main lobe signals or, in some instances, cause detrimental amplitude modulation of the main lobe signal. In many instances, the signal caused by the null steering device can have an amplitude substantially greater than the amplitude of the desired signal. Thus, it is essential that the main lobe signals, which are the desired signals for the main channel, be removed from the secondary antenna signals before the null steering apparatus operates on them.